An American woman suspected of spying is being investigated, China's Foreign Ministry said on Tuesday, as President Xi Jinping left for the United States on an official visit.

Sandy Phan-Gillis, of Texas, has been held by Chinese authorities for about six months and is suspected of spying and stealing state secrets, according to a statement from her family released this week.

The U.S. State Department said it was aware of her detention and monitoring her case.

Chinese Foreign Ministry spokesman Hong Lei said Phan-Gillis was suspected of "endangering China's national security" and is being investigated by "relevant Chinese authorities".

China has permitted her at least six consular visits, and she is in good health and cooperating with the investigation, Hong told a news briefing.

In a statement, State Department spokeswoman Katy Bondy said Phan-Gillis was arrested on March 20 and confirmed the U.S. consulate visits. It referred questions about her arrest and any charges to local authorities.

"We continue to monitor her case closely," Bondy said.

China's Ministry of State Security could not be reached for comment.

"Sandy is not a spy or a thief," her husband Jeff Gillis said in the family statement posted online. "She is a hard working businesswoman who spends huge amounts of time on non-profit activities that benefit Houston-China relations."

Phan-Gillis, who has Chinese ancestry and was head of the Houston Shenzhen Sister City Association, visited China on a trade delegation from Houston and was detained while attempting to cross from the southern city of Zhuhai to Macau on March 19, according to the family statement.

Phan-Gillis's husband did not immediately respond to messages seeking comment.

China's state secrets law is notoriously broad, covering everything from industry data to the exact birth dates of state leaders. Information can also be labelled a state secret retroactively.

Phan-Gillis is being held in the southwestern city of Nanning, the family said. She had been under house arrest until she was transferred to a detention centre on Saturday, it added.

It was unclear whether any formal charges have been brought. A lawyer working on her case could not be immediately reached.

"Sandy is in very poor health," the family statement said, adding that she suffers from high blood pressure, high cholesterol and high blood sugar. She has been hospitalised repeatedly while detained, it said.

While U.S. consular officials have been able to meet with Phan-Gillis, she has been unable to contact family and friends, the statement added.